RF power amplifiers are used in a wide variety of communications and other electronic applications. These amplifiers are made up of one or more cascaded amplifier stages, each of which increases the level of the signal applied to the input of that stage by an amount known as the stage gain. Ideally, the input to output transfer of each stage is linear; a perfect replica of the input signal increased in amplitude appears at the amplifier output. In reality, however, all RF power amplifiers have a degree of non-linearity in their transfer characteristic. This non-linearity results in the distortion of the output signal so that it is no longer a perfect replica of the input. This distortion produces spurious signal components known as intermodulation products. Intermodulation products are undesirable because they cause interference, cross talk, and other deleterious effects on the performance of a system employing RF power amplifiers. Accordingly, the prior art reflects various methods and devices designed to reduce the distortion produced during RF power amplifier operation. Two methods commonly suggested are predistortion and feed forward.
Predistortion utilizes an auxiliary distortion source that produces an auxiliary distortion signal similar to the distortion generated by a power amplifier. The auxiliary distortion signal is added to the power amplifier input in the correct gain and phase to promote cancellation of the distortion at the output of the power amplifier. This method requires matching the distortion characteristics of two dissimilar sources and hence limits the amount of correction which can be obtained.
The feed forward method does not have this limitation because it separates out that distortion generated by a power amplifier and adds it back into the power amplifier's output with gain, phase and delay adjusted for maximum cancellation. The amount of distortion reduction available using feed forward is limited only by the accuracy of the gain and phase adjustments and the correlation between the main amplifier and the error amplifier transfer functions.
Referring to FIG. 1A, there is shown a prior art feed forward system in block diagram form. Splitter circuit 12 divides the input signal on lead 11: one part is sent to power amplifier 14 and the other to cancellation circuit 18 via path 15. The output from power amplifier 14 includes a distortion component caused by the amplification of the input signal. A portion of the output signal from the power amplifier 14 is taken from directional coupler 16 and sent to cancellation circuit 18. The gain, phase and delay of the input signal on lead 15 is adjusted by fixed gain, phase and delay adjusters so that a portion of the input signal is cancelled when combined with the signal from directional coupler 16, to derive a distortion component on lead 19. The distortion component is adjusted by fixed gain, phase and delay adjusters, so that when the distortion component is combined with the power amplifier output, at directional coupler 10, the resultant output signal is free from distortion. The problem with this method, however, is the use of fixed gain, phase and delay adjuster which preclude the ability adjust gain and phase parameters in response to operating point changes, such as, for example, input signal variations, voltage variations, and temperature fluctuations.
Referring to FIG. 1B, there is shown yet another prior art feed forward system which attempts to overcome the above mentioned shortcomings. A test signal, or pilot, is injected, via coupler 30, into the main signal path of power amplifier 24. The magnitude of the pilot, when detected at the amplifier output, is used by automatic control circuit 32 to adjust the gain and phase of signals on lead 29 in order to eliminate both the pilot and the distortion introduced by the power amplifier 24. The problem with this approach is that the injected pilot signal occupies a portion of the system bandwidth that would otherwise be used by carriers, and therefore reduces the efficient use of system resources, which in turn adversely impacts system throughput. In addition, the embodiment in FIG. 1B still teaches the use of fixed gain, phase and delay adjuster to provide carrier cancellation.
Referring to FIG. 1C, there is shown yet another prior art feed forward system, designed to receive an input signal having at least one carrier signal therein in a prescribed frequency range. This input signal is applied to first and second circuit paths. The first circuit path has a power amplifier 110 that receives the input signal and produces an output signal with a distortion component. The second circuit path is designed to delay the input signal, without distortion. A portion of the signal from the first circuit path is combined with the delayed signal of the second circuit path to form a signal representative of the distortion generated by the power amplifier 110. Next, the signal representative of distortion is subtracted from the first circuit path output in order to cancel the distortion components therein.
In order to assure maximum distortion removal, a control circuit employing a narrow band scanning receiver scans the signal representative of distortion, over the prescribed frequency range, to locate carrier signals. Once a carrier signal is located, the magnitude of the detected carrier signal is supplied to controller 140 via narrow band receiver 150. Controller 140 then modifies the amplitude and phase parameters of amplitude and phase corrector 105 in order to drive the carrier component within the output of cancellation circuit 115 to a minimum. Thereafter, controller 140 scans the first circuit path output 132, over the prescribed frequency range, to detect intermodulation products. Once intermodulation products are found, the parameters of amplitude and phase adjuster 122 are modified by controller 140 to drive the intermodulation products appearing at the first circuit path output to a minimum.
The problem with this approach stems initially from its level of complexity. The process of scanning for frequencies representative of carrier signals or intermodulation products requires the use of highly selective scanning receiver circuitry which adds complexity and expense to feed forward error detection and correction circuitry. In addition to complexity, this approach suffers from an inherent inability to provide adequate carrier cancellation over a large system bandwidth, especially when two or more carriers are received simultaneously and require different phase and gain adjustments in order to be properly cancelled. Moreover, scanning techniques may be vulnerable to all types of correlated interference, such as, for example, co-channel interference and adjacent channel interference which may be mistaken for a desired signal and therefore cause the system to respond erroneously. This inherent weakness raises questions regarding scanning type feed forward correction circuits and their viability within an environment characterized by high levels of correlated interference.
It would be extremely advantageous therefore to provide a feed forward distortion minimization circuit that continuously, accurately and efficiently performs the gain and phase adjustments necessary to improve and maintain the intermodulation performance of a power amplifier, while avoiding the shortcomings of the prior art.